There are only few existing processes for preparing o-alkylfluorobenzenes which attract increasing interest as intermediates for active compounds. Blocking undesirable substitution positions by tertiary butyl groups permits the selective introduction of fluorine at aromatic nuclei by nitration, reduction, heating in the presence of fluoride ions (Baltz-Schiemann reaction) and removal of the butyl groups (J. Chem. Soc. Perkin Trans. I 1987, 1).
U.S. Pat. No. 5,306,849 discloses that some fluorobenzenes can be prepared from the corresponding chloroformates by heating in the presence of hydrogen fluoride and in the absence of solvents in yields of maximally 69%. Use of trichloro-trifluoroethane as solvent resulted in a significant worsening of the space-time yield and the conversion, even when the temperature was increased. Thus, 2,6-dimethylfluorobenzene could only be obtained in a yield of 40%.